This invention relates generally to an optical waveguide and a manufacturing method thereof, and more particularly, to an optical waveguide equipped with a grating, and a method of manufacturing said optical waveguide.
The grating in which a refractive index is periodically changed in a light transmitting medium for transmitting light in a confining state, conforms to an optical fiber or an optical waveguide, and has properties of being low loss, miniaturized and the like. Therefore, the grating is applied to various purposes such as a light-reflective device for a laser oscillator, a light-reflective device for a hybrid laser external oscillator combined with a semiconductor, wave separation and integration of optical signal for wavelength multiplexing, equivalent compensation of dispersion of light transmitted in the optical fiber, pulse compressing, equalization of wave length characteristics in a gain of a light amplifier, various sensors for measuring stress or temperature, observation of light-divided tracks, and so on. There are some kinds of gratings, such as a short-period grating in which the refractive index is periodically changed in a period same as the wavelength of the transmitted light, and a long-period grating in which the refractive index is periodically changed in a period longer than the wavelength of the transmitted light.
Further, a chirped grating in which a grating period is changed, is used for a wide band rejection filter, and an apodized grating is used for controlling side band. Thus, these various kinds of gratings including modified gratings have been developed as important optical materials.
Therefore, in addition to the formation of the grating devices in the optical fiber, there has been reported an attempt for miniaturizing and integrating a substrate by forming gratings in the optical waveguide.
Conventional gratings have been applied only to an optical fiber or optical waveguide in which a core containing germanium dioxide (GeO.sub.2) as a refractive index increasing element is included in silica (SiO.sub.2). That is, intensified light of ultraviolet pulse laser in wavelength from 200 to 300 nano-meters (nm) is applied to the waveguide comprised of the above-mentioned elements, thereby forming the grating by changing the induced refractive index.
However, the application of the ultraviolet radiation takes dozens of minutes, and gives small sensitivity of the changing of the refractive index, so an improvement has been demanded. In order to increase the changing of the refractive index by the application of the ultraviolet radiation, there has been proposed a method of keeping an optical fiber in a high pressure hydrogen atmosphere of hundreds of atmospheric pressure for about a few weeks for adding hydrogen to the fiber, or a method of adding boron (B) or tin (Sn) together with germanium (Ge). Owing to these methods, the sensitivity of changing of the refractive index has been increased.
In order to induce the changing of the refractive index by the application of the ultraviolet radiation, germanium (Ge) elements are essential. It has been considered that the change in the refractive index is caused by change in atomic state of the germanium element by the application of the ultraviolet, radiation. As mentioned above, the application of the ultraviolet radiation changes the refractive index by non-thermal equilibrium, but it gives no change in concentration of a germanium atom which is an impurity for increasing the refractive index in the optical fiber or optical waveguide.
The conventional method for applying the ultraviolet radiation has following problems. First, a pulse laser for applying the ultraviolet radiation takes highly accumulated energy and considerably long time for radiation, which increases its cost. Second, it takes long time to prepare materials and apply the ultraviolet radiation, which makes it difficult to manufacture in volume. Third, materials for forming the fiber or waveguide are limited. Fourth, the change in the refractive index by the application of the ultraviolet radiation is a temporary phenomenon by freezing the non-thermal equilibrium state, so that it may naturally return to its previous state by thermally thawing. In order to decrease change in the property in use at ordinary temperatures, and to secure long term reliability, it is required that a equipment be designed considering an initial property that amount of change in the refractive index after the application of the ultraviolet radiation is deteriorated in dozens of percentages by accelerative aging after forming a grating. For avoiding natural degeneration due to this thermal thawing, there has been proposed a method of using the change in the refractive index in the long-period grating, which is caused by relaxing the residual stress remaining inside the fiber drawn by pre-form combining pure silica core and fluorine dope cladding. However, this method of relieving the stress remaining inside the fiber also contains a problem of utilizing non-thermal equilibrium.